Feature Article

AUV Designed
For Marine Measurements
Small, Low-Cost Platform With ADCP, CTD for Water Profiling

Liu Jie

Liang Jie

CTD measurements during descent and ascent of the AUV in December 2011.

Designed to measure dynamic ocean environmental elements, a new AUV developed by the National Ocean Technology Center (NOTC) of China is a low-cost, small observation platform capable of underwater autonomous navigation and positioning. To measure current profile, CTD profile, water level and water depth, the AUV carries instruments such as an upward-looking acoustic Doppler current profiler (ADCP), down-looking ADCP and CTD.

Upon reaching a desired location, the AUV will sink and remain fixed in place on the seafloor to conduct continuous bottom-supported measurements. The AUV will then automatically ascend after a preset period and also conduct measurements during the return process.

When navigating on the surface, the AUV will transmit the acquired data to users via satellite. Users can obtain raw data from the AUV through a cable connection and process the data via NOTC software.

AUV Design
The AUV platform is 3.8 meters long and 0.6 meters high, with a diameter of 250 millimeters. There is a 100-millimeter-diameter, coupled ancillary chamber on each side of the torpedo-shaped main chamber. For stable bottom-supported measurement, the AUV adopts a tri-body structure of a main chamber and two slender ballast tanks, all connected by four support legs. The main chamber of the AUV consists of compartments for oceanographic measurements, a battery, communications and propulsion. For reliable connections and sealing, these compartments are connected and sealed by clamps and reinforcing ribs.

Upon reaching the preset observation position, the AUV's water tank will be injected with seawater and then sink. When the preset rising time is met, the AUV can automatically rise from the seafloor through takeoff-and-ejection-type load discarding.

Design factors such as the sensors' structure, layout, line shape and flow field analysis were taken into account. The design includes two main aspects. First, the CTD must be able to measure freshwater (if it cannot, the accuracy of measurement data could be affected) and have the ability to avoid bubbles caused by the vehicle's movement. However, simply installing the CTD on the AUV body would destroy the laminar flow field and increase the water resistance of the vehicle, so this had to be compensated in the design. A hydrokinetic fairwater cone was designed for the CTD, which was adapted to the nose shell of the AUV. Through this method, the effect of the CTD on the flow field is minimized.

Second, an ADCP needs a low-noise environment and immunity from the vehicle's navigation posture and movement-created bubbles. Like the CTD, the ADCP array would also increase the water resistance of the vehicle. To solve this problem, two fairwater cones were designed and installed around the upward and downward ADCPs; a design that proved to be effective in later experiments.

The AUV's control system adopts a field bus-based distributed structure and consists of a general control unit, a navigation control unit, a measuring and communication unit, a navigation and positioning unit, and a system-monitoring unit (black box). Each one controls and manages corresponding instruments, equipment and parts, and cooperates with each other to realize respective functions under the control of the general control unit.

The AUV has wired, wireless and satellite communications. Safety measures include forward-looking sonar to detect and avoid obstacles, an acoustic beacon to send sound signals for vehicle tracing in case of an accident and a black box to record work processes and states.

Workflow
To meet the measurement requirements, the AUV adopts the trapezoid profile measurement mode and the bottom-supported measurement mode. The former is named after the shape the AUV creates relative to the sea surface when cruising. A whole profile includes water surface navigation, descending operation, underwater horizontal navigation and ascending operation.

The tracking, mission and other information will be input through the deck unit before deployment. The AUV will then carry out both the trapezoid profile measurements and bottom-supported measurements according to the information.

The AUV will make several vertical trapezoid profiles along the route for cruising measurements. When floating in the water, it will conduct GPS positioning and pressure modification, and send the data via satellite. Upon reaching a preset position underwater, the water chamber will be injected and then sink to the bottom, where GPS positioning will be conducted again. Continuous bottom-supported measurement will begin when the AUV reaches the seabed. To continue this article please click here.

Liu Jie is a senior engineer at the National Ocean Technology Center of China. He received his bachelor's degree in computer application from Tianjin Normal University in 1994.

Liang Jie is a professor at the National Ocean Technology Center of China. He received a bachelor's degree in oceanography physics from Ocean University of China in 1984.

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